In recent years, the market for solid-state imaging devices has expanded remarkably following the wide use of the digital cameras and mobile phones with cameras. Furthermore, digital single-lens reflex cameras have been available more and more in which a wide variety of optical lenses, from wide-angle lens to telephoto lens, are interchanged and used. Even in such a situation, the strong demand for thinner digital cameras still exists. Stated differently, this means that the focal length of a lens used for the camera module becomes shorter, and that light enters a solid-state imaging apparatus with a wide angle (that is, an angle obtained when measuring from a vertical axis of an incident surface of the solid-state imaging device is wider).
In solid-state imaging devices such as CCD image sensors or CMOS image sensors, an imaging region includes a plurality of semiconductor integrated circuits (unit pixels) having light-receiving elements arranged in a two-dimensional array, and converts a light signal from an object into an electric signal. The sensitivity of the solid-state imaging device is defined based on an amount of output current of a light-receiving element to an amount of incident light. Therefore, leading the incident light surely into the light-receiving element is an important factor for improving the sensitivity.
FIG. 10 shows an example of a basic structure of a common unit pixel 300 in the conventional technique.
The unit pixel 300 includes, as shown in FIG. 10, a microlens 305, a color filter 302, an Al wire 303, a signal transmission unit 304, a planarization layer 308, a light-receiving element 306, and a Si substrate 307. In this configuration, incident light 356 (light indicated by dashed lines) which enters vertically into the microlens 305 is separated into colors using one of red (R), green (G), and blue (B) color filters 302, and then converted into an electric signal at the light-receiving element 306. For its relatively high light-collection efficiency, microlenses having the above configuration are used in almost all solid-state imaging devices.
However, since the solid-state imaging device includes a plurality of unit pixels arranged in two-dimensional array as described above, the incident angle of light entering the unit pixels leans as the unit pixel is farther from the middle portion (center portion) toward the peripheral portion of the imaging region. As a result, a problem is caused that the light-collection efficiency of the unit pixels in the peripheral portion decreases as compared with that of the unit pixels at the middle portion. For example, when the light is received by the unit pixel 300 of the conventional technique shown in FIG. 10, the incident angle of the light entering the unit pixel 300 is smaller at the middle portion of the imaging region (as the incident light 356 indicated by dashed lines), whereby almost all of the light is collected by the light-receiving element 306 and become effective light. In contrast, in the peripheral portion of the imaging region, the incident angle of the light entering the unit pixel 300 is greater in the peripheral portion of the imaging region (as the incident light 357 indicated by the solid lines), which causes the incident light 357 to be intercepted by the Al wire 303 in the unit pixel 300 and unable to reach the light-receiving element 306. Therefore, the light-collection efficiency is reduced.
In view of this, as shown in FIG. 11, the Al wire 303 and the light-receiving element 306 are shifted (shrank) in an outward direction (toward the edge) in the imaging region of the unit pixel 300 in the peripheral portion of the imaging region, in an attempt to improve the light-collection efficiency of the incident light 357 having a great incident angle.
Alternatively, as another measure for solving the above decrease in collection efficiency, a solid-state imaging device has been proposed which realizes a gradient index lens having effective refractive index distribution with its fine structure smaller than or approximately the same as the wavelength of incident light (for example, see Patent Literature (PTL) 1). Specifically, at the center portion of the imaging region in the solid-state imaging device, a gradient index lens having an effective refractive index distribution symmetrical to the center of the unit pixel is formed with a combination of a plurality of zones which is in a concentric structure and divided into line width approximately the same or shorter than the wavelength of the incident light. Furthermore, in the peripheral portion of the imaging region of the solid-state imaging device, the gradient index lens having the effective refractive index distribution asymmetrical to the center of the unit pixel is formed with a combination of a plurality of zones which is in a concentric structure and divided into line width approximately the same or shorter than a wavelength of incident light, and the center of the concentric structure is shifted (offset) from the center of the unit pixel. With this configuration, even when the light incident on the peripheral portion of the solid-state imaging device enters obliquely with a great angle relative to a vertical axis of the incident surface (light-receiving surface), the incident light can be collected at the light-receiving element and the sensitivity in the peripheral portion of the solid-state imaging element can be equivalent to that obtained at the middle portion. As a result, when the incident angle of the principal light ray from the optical lens of the camera is constant, the sensitivity can be prevented from being decreased.